According to Patent Specification WO90/11519 such devices can be of a kind comprising first and second liquid flow channels of porous materials leading from a respective pair of channel ends to a common site, which channels are operable to transfer liquid by capillary flow to the common site in sequentially timed manner following simultaneous application of the liquid to the pair of channel ends.
In one more particular form of this kind of device the first and second channels merge into, and transfer liquid in sequentially timed manner to a common channel. It follows that the later one of the first and second channels to transfer liquid to the common channel will itself also receive liquid from the other channel. The one channel accordingly receives two opposite flows which merge. After such merging it is normally appropriate for liquid flow to continue through the one channel in the same direction as the initial flow therein. However, continuing flow from the other channel can act undesirably against this result.